The proposed work is a continuation of our studies on the regulation of 3 processes in E. coli: 1. enzyme formation in the biosynthesis of arginine (an essential anabolic pathway); 2. initiation of plasmid replication (synthesis of a genetic element); enterotoxin synthesis (production of secreted proteins). Toward the first project, we have determined the nucleotide sequences of argR, the gene for the arginine repressor and we have isolated and characterized the repressor proteins from E. coli K12 and E. coli B. The 2 proteins differ by a single amino acid (leucide instead of proline), but this replacement confers a different type of regulation on the genes of the arginine pathway. In future studies we plan to study the interactions of the 2 regressors with their target sites on the genes of arginine biosynthesis (ARG boxes) and with the co- repressor L arginine. The arginine repressor has been shown to have a second function in the resolution of multimers of plasmids into monomers by site-specific recombination and we plan to analyze this function of the repressor. The resolution of multimers insures plasmid stability. For the second project we have analyzed the structure of several basic replicons of plasmids belonging to IncF incompatibility groups. We have determined the complete nucleotide sequence of RepFIC, a basic replicon present in the IncFI enterotoxin plasmid P307. We plan to study the capture and action of RepAl of RepFIC, a protein which is required for the initiation of replication. We also plan to study the functional elements of RepFIB, another basic replicon in P307, which seems to have a novel mechanism of replication control. For the third project, we have been studying the regulation of the formation of heat-labile enterotoxin (LT) and a heat-stable enterotoxin (STII). We plan to isolate and characterize mutants affecting the regulation of toxin production and to study sites in the structural genes of these toxins that are involved in the control of gene expression.